1. Field of the Invention
The present invention relates to the field of voltage converters of low voltage switched-mode power supply type. The present invention more specifically relates to isolated power supplies, that is, power supplies having no common point between the input voltage (for example, the A.C. supply system) and the regulated D.C. output voltage. The isolation is obtained by means of a transformer having a primary winding associated with a switch and a secondary winding associated with a diode and with a capacitor providing the output voltage. The present invention more specifically relates to so-called self-oscillating converters, in which the switch is controlled in frequency modulation, as opposed to converters having their switch controlled in pulse-width modulation.
2. Description of the Related Art
FIG. 1 shows a conventional example of a switched-mode power supply of the type to which the present invention applies. Two input terminals P, N receive an A.C. voltage Vac, for example, the mains voltage. Voltage Vac is rectified, for example by a fullwave rectification by means of a diode bridge 1. The A.C. input terminals of bridge 1 are connected to terminals P and N. Rectified output terminals 2, 3 of bridge 1 provide a voltage which is generally smoothed by means of a capacitor C1 connected between terminals 2 and 3. Terminals 2 and 3 form the input terminals of the actual switched-mode power supply.
The converter of FIG. 1 is a so-called “flyback” converter in which a transformer 4 with inverted phase points has its primary winding 5 connected in series with a switch 6 between terminals 2 and 3. The phase point of winding 5 is connected to a terminal of switch 6 and the other terminal of the winding is connected to terminal 2. A secondary winding 7 of transformer 4 is associated with a capacitor C2 across terminals 8, 9 of which D.C. output voltage Vout is provided. The phase point of winding 7 is connected to terminal 8 by a diode D2, the cathode of diode D2 being connected to terminal 8. The other terminal of winding 7 is connected to terminal 9.
When switch 6 is closed, the phase point of winding 7 is at a negative potential. Diode D2 thus is blocked and a current is accumulated in primary winding 5. Upon turning-off of switch 6, the phase points of windings 5 and 7 both become positive. Diode D2 is forward biased. Capacitor C2 is then charged with the power transferred to secondary winding 7.
Switch 6 is controlled by a circuit 10 (CTRL) having the function of cyclically turning off and turning on switch 6. In a self-oscillating converter, the turning-off of switch 6 is caused by comparison of the current flowing through this switch when on to a reference value. For this purpose, a detector 11 (for example, a resistor) in series with switch 6 is generally used, the result of its measurement being provided to circuit 10. To turn switch 6 on, the demagnetization of an auxiliary winding 7′ of transformer 4 is monitored. Auxiliary winding 7′ is in direct phase relation with secondary winding 7. Accordingly, a detection of the end of demagnetization of winding 7′ corresponds, as a first approximation, to a detection of the end of demagnetization of winding 7. The phase point of winding 7′ is connected to an input terminal of circuit 10 while the opposite terminal of the winding is connected to ground 3. To detect the end of demagnetization, the voltage across auxiliary winding 7′ is monitored by means of circuit 10. The demagnetization is considered as finished when this voltage falls under a predetermined voltage threshold. Switch 6 is then turned on. It should be reminded that, since the phase points of the primary and auxiliary windings are inverted, the voltage across auxiliary winding 7′ is negative out of these demagnetization periods (when switch 6 is on).
Generally, auxiliary winding 7′ is also used to provide a local supply voltage to control circuit 10. For this purpose, a local supply capacitor C3 is connected across the supply terminals of circuit 10. A positive electrode 12 of the capacitor is connected, by a diode D3, to the phase point of winding 7′, the anode of diode D3 being connected to the phase point. The other electrode of capacitor C3 is connected to ground 3.
FIG. 2 shows a conventional example of a self-oscillating control circuit 10 of a voltage converter. Switch 6 is generally formed of a MOS transistor having its drain connected to primary winding 5 and its source, at node 37, connected by a resistor R11 to ground 3. Resistor R11 has the function of a current-to-voltage converter for an input terminal of a first comparator 13 of circuit 10. Comparator 13 has the function of comparing the current in switch 6 with a reference value VRI provided to the other terminal of comparator 13. Reference voltage VRI of comparator 13 is chosen according to the desired output voltage Vout and to the transformation ratio between the primary and secondary windings. Voltage VRI conditions the converter power, which is proportional to the value of the inductances of transistor 4 and to the square of the current in primary winding 5 when switch 6 is on. The output of comparator 13 is sent onto the reset input (R) of an RS flip-flop 15 or the like, the non-inverted Q output of which provides the control signal of switch 6. The Q output of the flip-flop is generally applied to the gate of transistor 6 via a driver 16. Set input S of flip-flop 15 is connected to the output of a second comparator 17 having the function of detecting the end of demagnetization of auxiliary winding 7′. An input of comparator 17 receives a voltage reference VRV from an element 18. Value VRV is chosen to correspond to a voltage threshold under which the demagnetization is considered as finished. Ideally, voltage VRV is zero. The other input of comparator 17 is connected, by a diode D4, to the phase point of winding 7′, the anode of diode D4 being connected to this phase point. Given the connections of comparators 13 and 17 (the positive input of comparator 13 is connected to node 37 and the negative input of comparator 17 is connected to diode D4), the output of comparator 13 switches high when the current in switch 6, multiplied by the value of resistance R11, exceeds voltage VRI, while comparator 17 switches high when the voltage across auxiliary winding 7′ (neglecting the voltage drop in diode D4) becomes smaller than voltage VRV.
As soon as comparator 13 outputs a high state, this state holds the priority at the level of flip-flop 15, which provides a low output level. This causes the turning-off of switch 6, and thus a demagnetization beginning. As soon as the demagnetization begins, the output of comparator 13 switches low, switch 6 being off. When the end of demagnetization is detected by comparator 17, its output switches high. With the output of comparator 17 high and comparator 13 low, the non-inverted Q output of the flip-flop switches high. This turns on switch 6. This operation carries on cyclically.
It can be seen that the frequency of the turn-on cycles of switch 6 is variable and that the switching edges are directly caused by the detections performed by comparators 13 and 17. This is why this circuit is called a self-oscillating circuit. Since the turning-on of switch 6 can only occur after demagnetization, such a converter only operates in a so-called “discontinuous” mode, as opposed to converters operating in “continuous” mode, where the demagnetization may be incomplete at each switching cycle.
In FIG. 2, the rest of the switched-mode converter components have only been partially shown. The presence of a resistor R1 connecting local supply line 12 to terminal 2 has however been illustrated. The function of resistor R1 is to enable charge of capacitor C3, to power circuit 10 at the system starting. To illustrate the powering of circuit 10 from the voltage across capacitor C3, all elements (voltage references 14 and 18, comparators 13 and 17, flip-flop 15 and driver 16) have been shown with their respective supply terminals connected to terminals 12 and 3.
As compared to a switched-mode power supply operating in pulse-width modulation (PWM), a self-oscillating circuit has the advantage of a low cost. In particular, it is not necessary to provide an oscillator generating a sawtooth-shaped signal, with a modulation of its pulsewidth.
The inputs of flip-flop 15 may be associated with trigger circuits. Further, a delay element may be provided at the output of comparator 17, according to its responsiveness. A low responsiveness is then compensated for by increasing detection threshold VRV and by delaying the output signal.
Converters with self-oscillating control circuits are also known, which allow for a regulation of the output voltage. However, these converters impose a measurement of the voltage at the transformer secondary and, accordingly, a galvanic isolation element to transmit the measured value to the control circuit. This considerably increases the cost and is an often crippling disadvantage of this type of converter.
A converter with a self-oscillating control circuit is thus conventionally incompatible with a regulation of output voltage Vout, while maintaining a low cost.